These photographs point out several facts very clearly. One is that the alternations of currents in the telephone line, like the motions of the molecules of air of the original sound, are highly complex and are not, as musical tones are, regular recurrences of equal vibrations. They show also that any vowel sound may be considered to be a regular recurrence of certain groups of vibrations of different amplitudes and of different frequencies.
[CHAPTER III]
ELECTRICAL SIGNALS
Electric calls or signals are of two kinds: audible and visible.
Fig. 15. Telegraph Sounder and Key
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